SELENIUM  CELLS 

THE; 

CONSTRUCTION,  CARE  AND  USE 
OF  SELENIUM  CELLS  WITH  SPECIAL 
REFERENCE  TO  THE  FRITTS  CELL 


BY 

THOS.  W.  BENSON 


NEW  YORK 

SPON  &  CHAMBERLAIN,  120  LIBERTY  STREET 

LONDON 

E.  &  F.   N.  SPON,  LIMITED,  57  HAYMARKET,  S.  W. 
1919 


Copyright,  1919 
BY  THOMAS  W.  BENSON 


VAIL-BALLOU     COMPANY 
BINGHAMTON.AND   NEW  YORK 


APPRECIATIVELY 

DEDICATED 

TO 

J.  A.  STELTZER 


415285 


FOREWORD 

THE  lack  of  definite  information  relative  to 
the  construction  of  selenium  cells  has  led  the 
writer  to  put  in  this  form  the  results  of  some  of 
his  experiments.  The  method  described  was 
originated  by  Mr.  C.  E.  Fritts  but  the  appa- 
ratus used  was  developed  by  the  writer. 

Despite  the  fact  that  sensitive  cells  are  very 
difficult  to  construct  by  the  methods  in  vogue 
the  use  of  apparatus  described  practically 
eliminates  failures,  the  cells,  almost  without 
exception,  being  useful  for  one  purpose  or  an- 
other. 

This  is  by  no  means  the  last  word  on  the  sub- 
ject. Many  improvements  are  possible  and 
have  been  pointed  out  in  the  text.  Should 
these  instructions  but  serve  to  interest  others 
in  the  fascinating  study  they  will  have  served 
their  purpose. 

THOS.  W.  BENSON. 

Philadelphia,  Pa. 


CONTENTS 

CHAPTER  I 

PAGE 

SELENIUM,   THE   ELEMENT I 

Discovery,  Naming  and  Classification.  Where  Found 
and  Method  of  Extraction.  Three  Forms,  Amor- 
phous, Vitreous  and  Metallic. 

CHAPTER  II 

CONSIDERATION  OF  CELL  TYPES  AND  THEIR  CHARACTERIS- 
TICS         5 

Factors  in  Design.  Bildwell  Cell.  Application  of 
Selenium  and  Annealing.  Ruhmer  Cell.  Bell  and 
Taintor  Cell.  Mercadier  Cell.  Gripenberg  Cell. 
Theory  of  Operation. 

CHAPTER  III 

THE  CONSTRUCTION  OF  FRITTS  SELENIUM  CELL  ....     20 

Description  of  Hot  Press.  Accessories  for  Press. 
Other  Apparatus  Required  and  Supplies.  Preparing 
Copper  Plate.  Applying  Selenium.  Treating  in  Hot 
Press.  Application  of  Gold  Foil.  Assembling  Cell. 

CHAPTER  IV 
TESTING  AND  MATURING  SELENIUM  CELLS 32 

Two  States.  Testing  and  Maturing  Set.  Accessories 
for  Set.  Construction  of  Rheostat  Arm  for  Wheat- 
stone  Bridge.  Measurement  of  Cells  by  Bridge 
Method.  Tests  for  Light  Sensitiveness.  Proper 
Voltage.  Classification  of  Cells.  Testing  for  Polari- 


CONTENTS 

PAGE 

zation.  Treating  with  A.  C.  Raising  Resistance  of 
Cells.  Testing  for  Current  Generation.  Recon- 
struction of  Useless  Cells.  Measurement  of  Cells  by 
Substitution  Method.  Sealing  Cells. 

CHAPTER  V 

APPLICATIONS  OF  SELENIUM  CELLS 51 

Photometric  Applications.  Transmission  of  Speech 
Over  Beams  of  Light.  Automatic  Control  of  Light 
Buoys  and  Isolated  Lights.  As  Recorder  of  Sun- 
light. Astronomical  Applications.  Talking  Pictures. 
The  Phonoptican,  How  the  Blind  May  Read  by 
Sound.  Controlling  Mechanisms  at  a  Distance. 
Electric  Dog.  Use  in  Cable  Telegraphy.  Burglar 
Alarm.  Selenium  Batteries,  Current  Generators. 
Telephonic  Properties.  Effect  of  Different  Current 
Sources  on  Sensitiveness  of  Cells. 

CHAPTER  VI 
THE  CARE  OF  SELENIUM  CELLS    .  62 


LIST  OF  ILLUSTRATIONS 


FIG.  PAGE 

1.  Bildwell   Cell 7 

2.  Ruhmer  Cell 10 

3.  Bell  and  Taintor  Cell 12 

4.  Mercadier    Cell 14 

5.  Gripenberg  Cell " 15 

6.  Fritts  Cell 17 

7.  Hot  Press  with  Cell  in  Place 21 

8.  Details  of  Hot  Press  (elevation) 22 

8.  Details  of  Hot  Press  (plan) 22 

9.  Platen  for  Cell ;  and  Template  for  Applying  Selenium  24 

10.  Fiber  Pieces  for  Enclosing  Cell 26 

11.  Mode  of  Assembling  Cell 30 

12.  Testing  and  Maturing  Set 33 

13.  Layout  and  Wiring  Diagram  of  Testing  Maturing  Set  35 

14.  Connections  for  Buzzer  and  Induction  Coil  to  Produce 

Alternating  Current 36 

15.  Connections  for  Interior  of  Rheostat  Box     ....  38 

16.  Circuit  for  Wheatstone  Bridge  Measurement     ...  40 

17.  Circuit  for  substitution  Method  of  Measurement    .      .  47 

18.  Showing  Cell   ready   for  Assembly  with   Four  Com- 

plete Cells  in  background 49 


CHAPTER  I 

SELENIUM,  THE  ELEMENT 

OVER  a  century  ago,  1817  to  be  exact,  the 
Swedish  scientist  Berzelius  discovered  a  new 
element  in  the  lead  chambers  used  for  the  man- 
ufacture of  sulphuric  acid  by  roasting  iron  py- 
rites. Noting  its  resemblance  to  Tellurium, 
the  name  for  which  having  been  derived  from 
the  Greek  for  Earth,  Tellus,  he  named  the  new 
element  Selenium  derived  from  the  Greek  for 
Moon,  Selene.  The  ending  'urn  being  used  to 
indicate  a  metal  according  to  the  practice  of 
naming  newly  discovered  elements.  Although 
believed  to  be  a  metal  for  many  years,  the  chem- 
ical reaction  of  Selenium  resembles  that  of  sul- 
phur to  such  a  degree  that  it  is  now  accepted 
to  be  a  non-metal  in  its  amorphous  and  vitreous 
forms.  In  its  third  or  crystalline  state  it  has 
many  metallic  characteristics  and  in  this  form 
termed  metallic  selenium.  In  the  Periodic 
System  it  occupies  the  place  between  Tellurium 
and  Sulphur. 


2 


SELENiUM    CELL    CONSTRUCTION 


Designated  by  the  symbol  Se,  selenium  has 
been  found  in  all  parts  of  the  globe  in  small 
quantities,  chiefly  in  combination  with  copper, 
lead  and  silver  forming  selenides,  in  certain 
pyrites  and  occasionally  in  its  pure  state.  It 
was  found  in  metoric  iron  by  Warren  in  1909. 

An  idea  of  its  wide  distribution  may  be 
gained  from  the  following  table: 

Mineral  Composition  Location 

Sulphur   Selenide   in   natural 

sulphur  Lispau  Islands 

Eucarite          Selenide  of  silver  and  copper  Chili 

Crooksite        Selenide  of  silver,  copper  and  Norway  and 

thallium  Sweden 

Clauthalite      Selenide  of  lead  Germany 
Lehrbachite    Selenide  of  lead,  copper  and 

mercury  Germany 

Zorgite            Selenide  of  lead  and  copper  Germany 


The  element  is  obtained  commercially  as  a 
by-product  from  the  manufacture  of  sulphuric 
acid,  various  methods  of  extracting  it  from  the 
chamber  mud  being  employed.  The  usual  pro- 
cess is  to  heat  the  well  washed  chamber  mud 
with  potassium  cyanide  and  nitrate  to  obtain 
an  alkaline  selenate.  The  element  is  then  pre- 
cipitated with  hydrochloric  acid  or  sulphur  di- 
oxide. 


SELENIUM,    THE    ELEMENT  3 

Selenium  exists  in  three  well  defined  forms, 
Amorphous,  Vitreous  and  Metallic. 

Amorphous  Selenium.  This  form  is  ob- 
tained as  a  finely  divided  brick  red  precipitate 
when  sulphur  dioxide  is  passed  thru  selenic 
acid.  It  is  soluble  in  sulphuric  acid  and 
slightly  so  in  carbon  disulphide.  It  has  a  Sp. 
Gr.  of  4.26,  with  no  definite  melting  point,  sof- 
tening gradually  and  running  together  be- 
tween 80°  and  100°  C.  In  this  state  it  is  an 
insulator. 

Vitreous  Selenium.  When  the  amorphous 
selenium  is  heated  to  217°  C  and  rapidly  cooled 
the  vitreous  form  results.  It  is  now  a  red  vit- 
reous mass,  slightly  less  soluble  in  carbon  bi- 
sulphide. When  a  thin  film  is  held  up  to  the 
light  it  shows  blood  red  in  color.  Sp.  Gr.  4.28, 
Atomic  Weight  79.5.  This  form  is  practically 
an  insulator  having  a  resistance  of  6  x  io9 
ohms  per  Cu.  Cent,  at  75°  C  or  about  3.8  x  io10 
as  great  as  that  of  copper.  It  can  be  electri- 
fied by  friction.  Vitreous  selenium  has  no 
definite  melting  point  being  hard  and  brittle  at 
40°  C  and  softening  gradually  as  the  tempera- 
ture rises,  becoming  fluid  at  210°  C. 


4  SELENIUM    CELL    CONSTRUCTION 

Metallic  Selenium.  By  cooling  melted  vit- 
reous selenium  to  210°  C  and  holding  it  at  that 
temperature  for  a  short  time  the  metallic  form 
results.  The  element  is  now  a  black  glossy 
opaque  mass,  a  fair  conductor  of  electricity 
but  improving  greatly  under  the  influence  of 
light.  It  melts  between  217°  and  220°  C,  Sp. 
Gr.  4.788,  insoluble  in  carbon  disulphide  but 
will  dissolve  in  sulphuric  acid  to  form  a  green 
solution.  It  is  the  latter  form  that  is  used  in 
the  construction  of  selenium  cells. 

When  selenium  is  vaporized  by  heat  it  gives 
off  dark  brown  fumes  having  an  odor  similar 
to  rotting  cabbage.  These  fumes  are  poison- 
ous and  care  should  be  taken  that  they  are  not 
breathed  to  excess. 


CHAPTER  II 

CONSIDERATION  OF  CELL  TYPES  AND  THEIR 
CHARACTERISTICS 

A  selenium  cell  consists  essentially  of  two 
electrodes  of  brass  or  copper  bridged  by  a  thin 
layer  of  metallic  selenium.  When  connected 
into  a  circuit  with  batteries  and  other  apparatus 
the  current  flows  from  one  electrode  to  the 
other  thru  this  selenium  bridge.  Since  the  re- 
sistance of  the  selenium  to  an  electric  current 
depends  upon  the  amount  of  light  falling  upon 
it  the  flow  of  current  thru  the  cell  will  be  con- 
trolled by  the  brilliancy  of  the  illumination. 

Metallic  selenium  being  opaque,  the  light 
penetrating  but  i/5o,oooth  of  an  inch  as  cal- 
culated by  Marx,  it  is  necessary  that  the  sele- 
nium layer  be  extremely  thin  in  order  that  the 
light  may  affect  an  appreciable  proportion  of 
the  total  conducting  area.  This  condition  is 
never  reached  when  the  electrodes  lie  parallel 
to  each  other  with  the  selenium  between  them. 
However  by  arranging  the  electrodes  so  that 

5 


O  SELENIUM    CELL    CONSTRUCTION 

the  current  flows  at  right  angles  to  the  plane  of 
the  selenium  surface  we  can  cause  all  the  cur- 
rent to  flow  thru  the  light  affected  area.  This 
can  only  be  accomplished  by  making  use  of  a 
transparent  conductor  for  one  electrode. 

To  realize  the  importance  of  the  above  fac- 
tors a  description  of  the  various  types  of  cells 
developed  by  the  many  investigators  in  this 
field  will  be  of  great  assistance.  The  different 
workers  made  use  of  various  arrangements  of 
the  electrodes  but  the  cells  fall  into  certain 
classes.  These  types  have  been  named  after 
the  inventor  or  the  one  most  prominent  in  the 
work  on  them. 

"The  Bildwell  cell  is  possibly  the  best  known 
type.  It  is  made  by  winding  two  bare  wires 
of  copper,  brass,  german  silver  or  platinum  on 
a  sheet  of  mica  or  slate.  The  wires  are  spaced 
about  i/32nd  of  an  inch  apart.  The  size  of 
the  wire  is  of  little  importance,  the  usual  prac- 
tice being  to  use  #28  wire  on  a  form  measur- 
ing two  by  one  inches.  In  Fig.  i  is  shown  this 
type  of  construction  using  a  mica  form,  the 
wires  being  fastened  by  passing  them  thru  holes 
at  the  ends  of  the  sheet. 


CELL    TYPES  7 

The  selenium  is  applied  to  the  cell  by  melt- 
ing it  over  the  wires.  The  cell  is  laid  on  a 
mica  covered  copper  plate  supported  over  a 
bunsen  burner.  The  temperature  of  the  cell  is 
raised  to  the  point  where  a  stick  of  selenium 
when  touched  to  the  cell  melts.  The  entire 
surface  of  the  cell  is  coated  with  the  selenium 
in  a  very  thin  layer,  smoothing  out  the  lumps 
with  a  sheet  of  mica  or  a  steel  knife.  To  get 


FIG.  i.    BILDWELL  CELL 


a  satisfactory  coating  the  temperature  must  be 
regulated  closely,  if  too  low  the  selenium  turns 
grey  and  the  temperature  must  be  increased  to 
melt  it,  if  too  high  the  selenium  collects  in 
drops  due  to  surface  tension  and  is  as  difficult 
to  spread  as  mercury.  The  proper  state  is  a 
semi-fluid  condition  which  it  attains  at  220°  C 
when  it  can  be  easily  manipulated. 


8  SELENIUM    CELL    CONSTRUCTION 

When  a  satisfactory  surface  has  been  ob- 
tained the  cell  is  transferred  to  a  copper  plate 
to  cool  while  the  bunsen  burner  is  turned  down 
to  give  a  temperature  of  120°  C.  When  cool 
the  cell  is  replaced  on  the  hot  copper  plate  and 
allowed  to  heat  up  again.  Shortly  the  whole 
surface  will  turn  grey  in  color  due  to  the  sele- 
nium crystallizing.  The  temperature  is  now 
slowly  increased  till  the  selenium  shows  signs 
of  melting,  this  will  be  indicated  by  the  edges 
turning  black.  The  bunsen  burner  is  imme- 
diately withdrawn  and  the  edges  allowed  to  re- 
crystallize.  The  burner  is  turned  down  a 
trifle  and  replaced  under  the  hot  plate.  The 
cell  is  watched  carefully  for  signs  of  melting 
and  if  none  appear  it  is  left  so  for  three  or  four 
hours.  If  it  melts  again  the  burner  should  be 
further  lowered,  just  sufficient  to  keep  the  cell 
a  trifle  below  the  melting  point  of  the  selenium. 
The  cell  is  then  allowed  to  cool  by  lowering  the 
burner  by  small  amounts  extending  over  a  pe- 
riod of  an  hour.  This  prolonged  heating  and 
slow  cooling  is  known  as  annealing. 

After  the  above  treatment  the  cell  is  com- 
plete save  for  mounting.  The  usual  method  is 
to  mount  the  cell  in  a  small  wooden  box  fitted 


CELL    TYPES  9 

with  a  glass  window  to  admit  the  light,  leads 
being  brought  from  the  electrodes  to  two  bind- 
ing posts  mounted  on  the  box.  This  protects 
the  cell  from  moisture  and  dust. 

It  will  be  apparent  that  with  the  above 
method  of  construction  it  is  impractical  to  get 
the  extremely  thin  layer  of  selenium  necessary 
if  the  light  is  to  affect  a  relatively  large  pro- 
portion of  the  total  area.  This  will  be  even 
more  clear  from  an  examination  of  the  cross 
sectional  view  of  this  type  of  cell  as  shown  in 
Fig.  i.  Here  we  have  a  comparatively  thick 
layer  of  selenium  bridging  the  space  between 
the  wires.  Of  this  layer  only  the  thin  surface 
film  facing  the  light  drops  in  resistance  while 
the  interior  part  is  unaffected.  This  means 
that  should  the  surface  layer  drop  to  even 
i/5ooth  'of  its  dark  resistance  the  total  drop 
of  the  cell  would  be  much  less. 

The  Ruhmer  cell  is  similar  in  construction 
to  the  Bildwell,  differing  only  in  the  form  of 
support.  A  porcelain  or  glass  tube  is  used  to 
support  the  parallel  wires  as  shown  in  Fig.  2. 
When  porcelain  is  used  the  constructor  can  fas- 
ten the  wires  at  the  ends  by  slipping  them  into 


10 


SELENIUM    CELL    CONSTRUCTION 


slots  cut  with  a  hack  saw.  With  glass  some 
other  means  are  necessary  to  hold  the  wire 
while  winding. 

The  selenium  is  applied  to  the  cell  in  the 


FIG.  2.     RUHMER  CELL 


same  manner  as  the  Bildwell  and  then  an- 
nealed. The  method  of  mounting  the  cells  as 
devised  by  Ruhmer  is  worthy  of  mention.  The 
cell  unit  is  enclosed  in  a  glass  tube  and  then 


CELL    TYPES  II 

the  air  exhausted.  By  attaching  the  leads  to 
an  incandescent  lamp  base  a  very  convenient 
arrangement  results.  The  cell  is  well  protected 
from  all  external  influences  and  is  therefore 
more  staple  and  reliable.  This  is  perhaps  the 
most  important  improvement  in  this  type  of 
cell. 

We  have  in  the  Ruhmer  cell  conditions  al- 
most identical  to  that  in  the  Bildwell,  namely 
a  large  area  of  conducting  selenium  that  is  be- 
yond the  range  of  the  light  and  hence  not  af- 
fected thereby.  This  is  offset  to  a  certain  ex- 
tent by  the  large  area  exposed  to  the  light  as 
these  cells  can  be  employed  with  a  parabolic 
reflector  to  cause  the  light  to  fall  on  all  sides. 
This  type  of  cell  was  employed  by  Ruhmer  in 
his  experiments  with  the  Photophone.  He 
succeeded  in  transmitting  speech  for  a  distance 
of  four  miles  'Using  a  speaking  arc  at  the  trans- 
mitting station. 

The  cell  developed  by  Bell  and  Taintor  in 
their  experiments  is  rather  novel  in  the  ar- 
rangement of  the  electrodes.  As  shown  in 
Fig.  3  the  electrodes  take  the  form  of  brass 
disks  separated  by  thin  mica  disks  supported 
by  two  brass  rods,  the  whole  being  clamped  to- 
gether by  nuts  on  the  ends  of  the  rods.  The 


12 


SELENIUM    CELL    CONSTRUCTION 


disks  are  one  inch  in  diameter,  eighteen  or 
twenty  being  sufficient  for  a  small  cell.  By 
drilling  the  holes  in  the  disks  of  different  sizes 
and  assembling  them  as  shown  it  is  possible  to 
have  alternate  disks  connected  to  the  same  rod. 


ci  j 


r: 


FIG.  3.     BELL  AND  TAINTOR  CELL 


After  assembling  and  clamping  the  cell  skele- 
ton is  chucked  in  a  lathe  and  the  surface  turned 
smooth  and  polished. 

The  selenium  is  applied  by  heating  the  form 
and  melting  it  on,  by  rolling  the  cell  back  an4 


CELL    TYPES  13 

forth  over  the  hot  plate  it  is  possible  to  get  an 
extremely  thin  film  of  selenium  on  the  smooth 
surface  offered  by  the  cell.  The  coating  is 
then  annealed  in  the  regular  manner.  This 
cell  can  well  be  mounted  in  a  glass  tube  and 
the  air  exhausted. 

The  advantage  gained  by  this  form  of  con- 
struction is  the  thin  film  of  selenium  obtainable 
due  to  there  being  no  spaces  between  the  elec- 
trodes into  which  the.  selenium  can  flow.  A 
cross  section  of  this  type  is  given  in  the  illus- 
tration. It  will  be  seen  that  although  the  main 
defect  of  the  cells  mentioned  previously  has 
been  reduced  to  some  extent  still  it  has  not 
been  removed  entirely. 

The  Mercadier  cell  is  similar  in  many  re- 
spects to  the  Bell  but  is  easier  of  construction. 
Two  strips  of  thin  copper  one  half  inch  wide 
are  wound  into  a  spiral  being  separated  from 
each  other  by  strips  of  mica.  One  face  of  the 
flat  spiral  is  filed  flat  and  then  polished.  The 
selenium  is  melted  onto  the  cell  and  then 
smoothed  off  with  a  strip  of  mica.  The  cell 
then  being  annealed  as  described  previously. 

We  have  here  a  condition  analogous  to  that 
in  the  Bell  cell,  the  only  advantage  being  rug- 


14  SELENIUM    CELL    CONSTRUCTION 

gedness  and  simplicity  against  a  loss  in  active 
surface  area.  For  experimental  purposes  this 
cell  is  entirely  satisfactory  for  if  it  does  not 
prove  sensitive  the  selenium  coating  can  be 
filed  off  and  another  applied. 


FIG.  4.    MERCADIER  CELL 


We  come  now  to  the  consideration  of  cells 
wherein  the  current  flows  at  right  angles  to 
the  surface  of  the  selenium.  This  implies  the 
use  of  at  least  one  electrode  on  the  surface  of 
the  selenium.  The  Gripenberg  cell  has  both 
electrodes  on  the  surface.  As  shown  in  Fig.  5 
the  electrodes  are  made  by  depositing  a  thin 
film  of  gold  on  a  glass  plate  and  with  a  sharp 


CELL    TYPES  I 5 

tool  removing  narrow  strips  of  it  to  form  a 
grid,  alternate  bars  of  which  are  connected  to 
the  same  terminal.  The  grid  arrangement  is 
shown  in  the  detail  illustration. 

The  selenium  is  not  applied  to  the  grid  in 
the  molten  state.     A  thin  plate  of  metallic  sele- 


PRESSURE  SCREW 


SELENIUM  PLATE 


CROSS  SECTION   OF  ASSEMBLED  CELL 


, GLASS  PLATE 


Milt 


'GOLD  FOIL  GRID. 


FIG.  5.     GRIPENBERG  CELL 


nium  is  obtained  by  melting  the  metal  on  a 
glass  plate  and  then  applying  pressure  with  a 
cold  glass  plate.  After  annealing  the  sele- 
nium will  be  found  to  adhere  closely  to  the  hot 
plate  in  a  thin  film.  The  cell  is  then  assembled 
by  placing  the  plate  with  the  gold  grid  in  a 
frame  having  a  portion  of  one  side  cut  out  to 


l6  SELENIUM    CELL    CONSTRUCTION 

form  a  window.  The  plate  with  the  selenium 
adhering  is  placed  over  the  grid  and  forced 
into  contact  with  same  by  means  of  a  small 
screw. 

A  study  of  the  cross  section  of  this  cell 
shows  that  the  current  inflowing  from  one  elec- 
trode to  the  other  must  pass  the  surface  of  the 
selenium.  Since  the  gold  film  is  semi-trans- 
parent the  light  passes  thru  it  to  affect  the  se- 
lenium and  will  have  a  maximum  effect  upon 
the  resistance  of  the  cell.  This  object  is  at- 
tained at  the  expense  of  loss  of  illumination 
since  the  gold  film  cuts  off  all  but  the  green  rays 
of  light.  The  advantage  just  mentioned  more 
than  outweighs  this  loss.  Could  a  trans- 
parent conductor  be  found  this  defect  would  be 
removed  entirely. 

The  Fritts  Cell  is  little  known  except  by 
name  but  is  superior  to  the  others  both  in  sim- 
plicity of  construction  and  correctness  of  de- 
sign. In  this  case  the  selenium  is  melted  di- 
rectly on  a  copper  plate  that  serves  as  one  elec- 
trode. While  soft,  pressure  is  applied  by  a 
non-adherent  plate  to  obtain  the  thin  film  neces- 
sary. The  selenium  enters  into  chemical  com- 


CELL    TYPES  I/ 

bination  with  the  copper  and  adheres  firmly  to 
it.  After  cooling  the  selenium  film  is  covered 
with  gold  leaf  to  form  the  other  electrode. 
The  cell  unit  can  be  mounted  between  two 
strips  of  fibre  as  shown  in  Fig.  6,  a  sheet  of 
very  thin  mica  serving  as  a  protection  to  the 
gold  foil  surface. 


COPPER  PLATE 
SELENIU 
GOLD  FOIL 


LIGHT  RAYS 


CROSS  SECTION   OF  ASSEMBLED  CELL 


COPPER  PLATE 
SELENIUM- 
GOLD  FOIL- 


FIG.  6.    FRITTS  CELL 

We  have  here  the  ideal  condition.  All  the 
current  must  flow  thru  the  light  affected  area 
in  passing  from  one  electrode  to  the  other. 
The  light  passes  thru  the  semi-transparent  gold 
foil  to  effect  the  change  in  resistance_of  the 
selenium  fikn.  ^The  only  disadvantage  lies  in 
the  diminishing  of  the  light  strength  by  the 


iS  SELENIUM    CELL    CONSTRUCTION 

gold  film.  Despite  this  however  the  Fritts  cell 
has  proved  the  most  sensitive  cell  made  having 
in  one  case  a  ratio  of  337  to  i,  that  is,  the  re- 
sistance in  the  light  is  but  %oths  of  one  per  cent 
of  that  in  the  dark. 

The  six  types  of  cells  described  cover  all  the 
types  worthy  of  special  mention  and  will  en- 
able one  to  select  a  cell  for  experimental  pur- 
poses. In  view  of  the  fact  that  the  Gripenberg 
and  Fritts  cells  are  superior  in  point  of  design 
the  selection  of  the  cell  becomes  a  question  of 
mechanical  difficulties  to  be  overcome.  The 
construction  of  the  grid  in  the  Gripenberg  type 
is  rather  difficult  unless  an  engraving  machine 
is  obtainable  whereas  the  Fritts  cell  requires 
very  simple  apparatus  for  its  construction. 

A  factor  often  overlooked  in  considering  the 
design  of  selenium  cells  is  the  relation  that 
light  and  electricity  bear  to  each  other.  These 
are  manifestations  of  the  same  force  and  their 
interaction  can  be  taken  advantage  of  in  the 
Fritts  cell  due  to  it  being  possible  to  cause  the 
current  to  flow  in  the  same  or  opposite  direc- 
tion to  that  of  the  light  vibrations.  The  im- 
portance of  this  will  be  covered  in  detail  later. 

No  mention  has  been  made  of  just  how  the 
light  affects  the  selenium  to  reduce  its  resist- 


CELL    TYPES  1 9 

ance.  This  is  still  a  moot  point,  one  theory 
being  that  the  light  being  electromagnetic  in 
nature  causes  the  molecules  of  the  selenium  to 
cohere  in  a  manner  similar  to  that  of  the  radio 
coherer  used  in  the  early  days  of  radio  teleg- 
raphy. However  the  conduction  thru  a  sele- 
nium cell  is  similar  to  that  in  an  electrolyte  and 
differs  from  metallic  conduction.  Consider- 
ing this  the  light  may  act  to  ionize  the  selenium 
in  some  manner  and  make  possible  the  more 
rapid  interchange  of  the  ions  from  the  opposite 
terminals.  Although  a  thorough  understand- 
ing of  the  action  taking  place  may  lead  to  the 
improvement  of  selenium  cells  it  is  not  within 
the  scope  of  the  present  work  to  consider  the 
various  phases  of  this  part  of  the  problem,  con- 
fining itself  as  it  does  more  to  the  practical 
production  of  the  cells. 


CHAPTER  III 

THE  CONSTRUCTION  OF  FRITTS  SELENIUM 
CELL 

THE  secret  of  the  successful  construction  of 
the  Fritts  selenium  cell  lies  entirely  in  the 
method  of  applying  the  selenium  to  the  copper 
plate.  The  selenium  is  melted  on  the  plate  and 
pressure  applied  at  the  same  time,  due  to  the 
simultaneous  action  of  heat,  chemical  affinity 
and  pressure,  crystallization  takes  place  and 
makes  prolonged  annealing  unnecessary.  One 
side  of  the  selenium  layer  enters  into  chemical 
combination  with  the  copper  plate  forming  a 
selenide  while  the  other  is  uncombined  result- 
ing in  a  film  that  is  polarized  or  has  different 
electrical  and  physical  conditions  at  front  and 
back. 

To  construct  the  cells  use  is  made  of  what 
may  be  termed  a  hot  press.  This  is  simply  a 
device  for  applying  pressure  to  the  selenium 
film  while  in  a  molten  state.  At  Fig.  7  is  given 
a  photograph  of  the  apparatus  while  Fig.  8 


20 


FRITTS    SELENIUM    CELL 


21 


22  SELENIUM    CELL   CONSTRUCTION 


FIG.  8.    DETAILS  OF  HOT  PLATE  PRESS  (ELEVATION) 


FIG.  8.    DETAILS  OF  HOT  PLATE  PRESS   (PLAN) 


FRITTS    SELENIUM    CELL  23 

shows  the  various  details  of  the  press.  The 
dimensions  can  be  changed  if  desired  but  the 
press  should  be  capable  of  exerting  a  pressure 
of  50  pounds  on  the  bed. 

A  base  of  any  convenient  size,  say,  16x8 
inches,  has  fastened  to  one  end  a  tripod  or  sim- 
ilar elevated  support.  A  slab  of  slate  4x8 
inches  and  %th  of  an  inch  thick  is  drilled  at 
the  ends  to  take  the  bolts  holding  the  pivot  and 
guide  posts  shown. 

The  posts  are  made  from  %cth  inch  strap 
iron  %  inch  wide.  The  pivot  post  being  made 
by  giving  the  metal  strip  a  quarter  turn  with 
a  large  wrench  while  holding  one  end  of  the 
strip  in  a  vise.  The  guide  post  is  formed  by 
slotting  the  end  of  the  bent  strip  to  pass  the 
lever. 

The  lever  is  made  from  %  inch  iron,  %  inch 
wide  and  20  inches  long.  One  end  is  drilled 
to  pass  'the  bolt  pivoting  it  to  the  pivot  post. 
A  saddle  bent  from  Vs  inch  sheet  iron  is  pivoted 
to  the  lever  as  shown  so  as  to  be  directly  over 
the  center  of  the  slate  slab.  The  saddle  serves 
to  equalize  the  pressure  at  point  of  contact  with 
the  cell  undergoing  treatment  and  gives  a  di- 
rect vertical  thrust. 

The  weight  used  with  the  press  is  made  by 


24  SELENIUM    CELL    CONSTRUCTION 

pouring  lead  into  a  round  form  2/4  inches  in  di- 
ameter and  5  inches  high.  Any  other  weight 
of  eight  pounds  can  be  used.  A  hook  bent 
from  a  strip  of  brass  or  iron  is  inserted  in  the 
lead  while  in  a  molten  state  and  serves  to  hang 
the  weight  on  the  lever. 


i 


FIG.  9.     PLATEN  FOR  CELL 


T 


FIG.  9.    TEMPLATE  FOR  AP- 
PLYING  SELENIUM 


A  bunsen  burner  is  utilized  to  heat  the  slate 
slab.  Some  convenient  means  should  be  em- 
ployed to  control  the  height  of  the  flame,  either 
a  valve  mounted  on  the  base  or  a  screw  clamp 
on  the  rubber  hose  supplying  the  gas. 

The  cell  while  undergoing  treatment  rests  on 
a  small  iron  block,  the  details  of  which  are 


FRITTS    SELENIUM    CELL  25 

given  in  Fig.  9.  The  block  is  i  %  inches  square 
and  %  inch  thick.  A  hole  is  drilled  at  one  side 
to  take  the  bulb  of  a  chemical  thermometer. 
This  block  serves  two  purposes,  it  allows  of  ac- 
curate determination  of  the  cell  temperature 
and  simplifies  the  assembly  of  the  cell  for  treat- 
ment. The  thermometer  should  have  a  range 
up  to  220°  C. 

This  completes  the  hot  press  proper.  In 
addition  will  be  required  a  2  or  4  oz.  mortar 
and  pestle  to  powder  the  selenium,  a  pair  of 
tweezers,  a  small  palette  knife  and  a  template 
for  applying  the  selenium  to  the  copper  plate. 
The  latter  is  made  from  a  sheet  of  mica  or 
thin  cardboard  with  rectangular  hole  cut  in  the 
center  as  shown  in  Fig.  9.  This  sheet  is  glued 
to  two  small  strips  of  wood  or  fibre.  The  tem- 
plate just  fits  over  the  copper  plates  and  con- 
fines the  selenium  powder  to  the  center  of  the 
plate. 

The  supplies  required  are  chemically  pure 
selenium,  specify  electrolytic  selenium  for  elec- 
trical purposes  when  ordering  and  the  proper 
material  will  be  supplied.  It  comes  in  the  form 
of  small  black  sticks  and  looks  like  sealing 
wax.  A  book  of  gold  leaf  as  used  by  decorat- 
ors is  required  for  the  front  electrode  of  the 


26 


SELENIUM    CELL    CONSTRUCTION 


cell.  The  patented  form  in  which  the  foil  is 
attached  to  a  sheet  of  thin  paper  is  the  easiest 
to  handle.  The  copper  plates  for  the  cell  are 
cut  from  Vs  inch  copper  or  brass  i  %  inches  long 
and  i  inch  wide.  These  plates  should  be  per- 
fectly flat  and  the  edges  free  from  burrs. 


t 

b" 

o 

1 

-*^K 

t 

2 

«" 

l" 

J-3^-, 

1 

0 

, 

t  _ 

o 

FIG.  10.     FIBRE  PIECES  FOR  ENCLOSING  CELL 


Some  clear  mica  will  be  required  for  the  front 
of  the  cell  to  protect  it  from  dust  and  moisture. 

The  fibre  sheets  to  enclose  the  finished  cell 
are  shown  in  Fig.  10.  These  are  cut  to  the  di- 
mensions shown  from  Vs  inch  fibre. 

The  construction  of  a  cell  is  accomplished  in 
the  following  manner.  The  copper  plate  is 


FRITTS    SELENIUM    CELL  2/ 

first  thoroughly  cleaned  with  fine  sandpaper 
and  polished.  Coat  the  plate  with  a  suitable 
flux  and  heat  in  the  bunsen  flame,  flowing 
solder  over  the  plate  to  tin  it.  While  the  solder 
is  still  molten  throw  off  as  much  of  the  solder 
as  possible  and  quickly  wipe  the  cell  with  a  dry 
rag.  This  should  result  in  a  thin  even  film 
of  solder. 

Grind  the  selenium  up  into  a  fine  powder  in 
the  mortar  and  place  in  a  well  stoppered  bot- 
tle to  protect  from  dust  and  moisture. 

Lay  the  tinned  copper  plate  on  the  table, 
tinned  side  up  and  place  the  template  over  it. 
With  the  palette  knife  place  a  small  quantity 
of  the  selenium  on  the  plate  and  smooth  it 
out  into  an  even  layer  %2  of  an  inch  thick. 
Remove  the  template  carefully  so  as  not  to  dis- 
turb the  selenium  and  lay  the  plate  on  one  end 
of  the  iron  block  having  the  thermometer  well 
in  it.  Cover  the  selenium  with  a  sheet  of  mica 
and  lay  another  smooth  block  %  inch  thick  on 
top  of  the  mica  sheet.  The  whole  is  conveyed 
to  the  hot  press  and  placed  in  the  center  of  the 
slate  slab,  the  lever  being  let  down  into  place 
with  the  saddle  centering  over  the  cell. 

Place  the  thermometer  in  the  well,  hang  the 
weight  on  the  lever  and  light  the  bunsen  burner, 


28  SELENIUM    CELL    CONSTRUCTION 

adjusting  it  so  an  even  blue  flame  results  with- 
out roaring. 

The  temperature  as  indicated  by  the  ther- 
mometer will  rise  slowly  due  to  the  bulk  of  the 
slate  to  be  heated.  At  150°  C  the  selenium 
will  soften  and  the  lever  settle  slightly.  Con- 
tinue the  heating  till  the  temperature  reaches 
220°  C  and  then  remove  the  bunsen  burner. 

The  cell  is  allowed  to  remain  in  the  press  un- 
til the  temperature  has  dropped  to  60°  C  when 
the  lever  can  be  lifted  and  the  cell  taken  out. 
The  mica  will  come  off  without  trouble  leaving 
a  thin  even  film  of  metallic  selenium  with  a 
shiny  grey  surface  adhering  to  the  plate. 

Now  go  around  the  edges  of  the  cell  with  a 
knife  or  small  file  and  remove  all  selenium 
that  may  have  flowed  over  the  edges  of  the 
plate  and  brush  the  cell  to  remove  all  traces 
of  dust  and  particles  of  selenium  and  copper. 
Lay  the  cell  face  up  on  a  sheet  of  glass  and 
flood  the  surface  with  alcohol  with  an  eye 
dropper  or  pippette.  Having  cut  a  strip  of 
gold  foil  I  inch  wide  lay  it  on  the  cell  and 
smooth  out  any  creases  with  the  fingers.  Now 
go  over  the  entire  surface  of  the  cell  with  the 
finger  tips  pressing  rather  hard  on  the  paper 
backing  on  the  foil.  Continue  this  till  the  alco- 


FRITTS    SELENIUM    CELL  2C) 

hol  that  seeped  through  the  paper  has  evap- 
orated. Then  flood  the  back  of  the  paper  and 
repeat  the  pressing.  When  the  second  appli- 
cation of  alcohol  has  dried  the  paper  can  be 
lifted  from  the  cell  without  difficulty  leaving 
the  foil  on  the  selenium.  Should  any  of  the 
foil  adhere  to  the  paper  replace  it  and  treat 
again  with  alcohol  and  pressure. 

The  above  applies  to  the  patent  foil.  If  the 
foil  is  loose  it  can  be  cut  by  placing  a  sheet  be- 
tween two  sheets  of  paper.  A  camel  hair 
brush  is  rubbed  through  the  hair  to  electrify  it 
and  the  foil  picked  up  by  touching  the  brush  to 
it.  The  foil  is  laid  on  the  cell  wet  with  alco- 
hol arid  smoothed  out  with  the  brush.  On  dry- 
ing the  foil  will  adhere  to  the  selenium. 

When  a  good  foil  surface  is  obtained  go  care- 
fully around  the  edge  with  a  small  knife  and 
remove  any  foil  that  extends  over  the  edge  of 
the  selenium  film,  otherwise  direct  contact 
might  be  made  between  the  foil  and  copper 
plate,  thus  short  circuiting  the  cell. 

Now  take  a  strip  of  paper  Vs  inch  wide  and 
coat  one  side  lightly  with  thick  shellac.  Wrap 
the  paper  around  one  end  of  the  cell  as  shown 
in  Fig.  ii.  A  second  sheet  of  paper  is  placed 
over  the  end  of  the  cell  and  attached  in  the 


30  SELENIUM    CELL    CONSTRUCTION 

same  manner.  The  paper  prevents  the  cop- 
per plate  coming  in  contact  with  the  terminal 
leading  to  the  gold  foil  as  will  be  evident  when 
the  cell  is  to  be  assembled  in  the  fibre  strips. 

The  method  of  completing  the  cell  assembly 
is  shown  in  Fig.  n.  Take  the  sheet  of  fibre 
with  the  hole  cut  in  it  and  place  a  %2  bolt  i 
inch  Ipng  in  one  of  the  holes.  A  sheet  of  thin 


LONG  STRIP 
-CELL  BACK 


PAPER 
MICA 
SHORT  STRIP 


CELL  WITHOUT 
BACK 


FIG.  u.    MODE  OF  ASSEMBLING  CELL 


clear  mica  is  laid  on  the  fibre  and  short  strip  of 
tinfoil  slipped  over  the  bolt.  A  nut  is  then  run 
on  to  clamp  the  tinfoil  and  make  good  contact. 
The  tinfoil  should  be  cut  off  even  with  the  edge 
of  the  opening  in  the  fibre.  Another  bolt  is 
put  into  the  other  hole  and  a  longer  strip  of 
tinfoil  clamped  to  it  also  by  means  of  a  nut. 
Turn  back  the  long  strip  of  foil  and  lay  the 


FRITTS    SELENIUM    CELL  31 

cell  between  the  bolts  with  the  paper  wrapped 
end  nearest  the  post  with  the  short  tinfoil 
strip.  The  paper  will  prevent  contact  between 
the  plate  and  this  post  but  make  sure  that  the 
tinfoil  strip  makes  contact  with  the  gold  foil. 
The  long  strip  of  foil  is  laid  over  the  back  of 
the  plate  and  the  other  fibre  strip  dropped  over 
the  bolts.  Nuts  are  then  run  on  to  clamp  the 
whole  together. 

The  cell  is  now  complete  and  should  be 
tested  as  described  in  the  following  chapter 
before  sealing  the  edges  with  sealing  wax. 


CHAPTER  IV 
TESTING  AND  MATURING  SELENIUM  CELLS 

The  Fritts  Cell  as  mentioned  by  the  inventor, 
when  first  made  has  one  of  two  states  or  con- 
ditions. In  one  the  resistance  is  very  high,  in 
the  other  very  low,  being  but  a  few  ohms  or 
a  fraction  of  an  ohm.  In  the  latter  state  it  is 
insensitive  and  possesses  no  definite  character- 
istic until  matured.  This  maturing  will  come 
about  gradually  if  the  cell  is  used  in  experi- 
menting but  can  be  greatly  hastened  by  the 
proper  treatment.  By  subjecting  them  to  an 
alternating  or  pulsating  current  the  resistance 
can  be  increased  rapidly. 

To  facilitate  the  maturing  and  enable  the  re- 
sistance of  the  cell  to  be  determined  at  various 
stages  of  the  treatment  the  maturing  and  test- 
ing set  shown  in  Fig.  12  can  be  made  use  of. 
The  apparatus  allows  of  the  measurement  of 
the  cell  by  the  Wheatstone  bridge  and  substi- 
tution methods  both  with  the  cell  lighted  and 
dark  and  permits  the  application  of  alternating 

32  ' 


TESTING   AND    MATURING 


33 


34  SELENIUM    CELL    CONSTRUCTION 

current  as  desired.  It  consists  of  a  base  on 
which  is  conveniently  mounted  a  closed  lamp 
box  to  light  the  cell  when  necessary  and  the 
various  switches  and  binding  posts  arranged 
for  quick  manipulation  of  the  circuit. 

The  details  of  the  device  are  given  in  Fig. 
13.  The  base  measuring  20  by  12  inches  has 
mounted  at  the  rear  center  a  wooden  box  con- 
taining a  75  Watt,  type  C,  Mazda  lamp. 
Where  lighting  current  cannot  be  obtained  a  12 
volt  automobile  headlight  bulb  may  be  used  to 
illuminate  the  cell.  The  lamp  in  either  case  is 
connected  to  the  switch  mounted  just  to  the  left 
of  the  box. 

The  front  of  the  lamp  housing  has  a  hole 
measuring  i  by  %  inches  cut  in  it  on  a  level 
with  the  lamp  filament.  Two  clips  cut  from 
spring  brass  of  the  shape  shown  in  the  detail 
drawing  are  mounted  on  the  front  of  the  box 
in  such  a  position  that  when  the  posts  on  the 
back  of  the  cell  are  slipped  into  the  holes  in  the 
ends  of  the  strips  the  window  of  the  cell  will  be 
opposite  the  opening  in  the  lamp  housing. 
Connection  is  made  to  the  cell  by  means  of  these 
clips. 

At  the  extreme  left  of  the^base  a  double  pole 
fuse  switch  is  mounted  to  control  the  alternat- 


TESTING   AND    MATURING 


35 


3D  SELENIUM    CELL    CONSTRUCTION 

ing  current  to  treat  the  cell.  Instead  of  fuses 
two  tubular  incandescent  lamps  are  screwed 
into  the  receptacles  on  the  switch  to  limit  the 
A.  C.  to  a  value  that  will  not  endanger  the  cell 
by  overheating  it.  Should  lighting  current  not 
be  available  use  can  be  made  of  a  buzzer  and 
telephone  induction  coil  connected  to  a  dry  cell 
as  shown  in  Fig.  14  to  furnish  current  for  treat- 
ing the  cell. 


TO 

TESTING 
SET 


TELEPHONE 

INDUCTION 

COIL 


FIG.  14.     CONNECTIONS  FOR  BUZZER  AND  INDUCTION  COIL  TO 
PRODUCE  ALTERNATING  CURRENT 


A  small  two  point  battery  switch  and  four 
double  spring  binding  posts  are  also  mounted 
on  the  base  and  wired  as  shown  by  the  dotted 
lines.  A  fairly  sensitive  galvonometer  is  em- 
ployed to  indicate  when  the  bridge  is  balance. 
A  telephone  receiver  might  be  used  for  the  pur- 
pose, opening  and  closing  the  circuit  to  cause 


TESTING   AND    MATURING  37 

clicking  in  the  receiver,  the  bridge  being  bal- 
ance when  the  noise  is  reduced  to  a  minimum. 
The  small  center  zero  ammeters  with  the  shunt 
removed  make  excellent  galvonometers. 

Ratio  arms  and  a  rheostat  box  having  a  max- 
imum resistance  of  100,000  ohms  complete  the 
apparatus  necessary  to  make  the  tests  on  the 
cells.  A  laboratory  set  with  this  range  is 
rather  expensive  and  since  our  measurements 
need  not  be  extremely  accurate  a  good  resis- 
tance box  may  be  made  from  resistance  units 
as  used  for  motor  starting  and  signal  work. 
These  units  consist  of  an  iron  tube  covered 
with  asbestos  on  which  is  wrapped  the  resis- 
tance wire,  the  wire  in  turn  being  covered  with 
a  vitreous  insulating  material  baked  in  place. 
These  units  can  be  purchased  quite  reasonable 
from  any  large  electrical  supply  house  in  any 
resistance  up  to  150,000  ohms.  For  our  pur- 
pose 15  units  will  be  required,  five,  200  ohm; 
five,  2000  ohm  and  five,  20,000  ohm ;  all  tapped 
at  the  center,  for  the  rheostat  arm.  For  the 
ratio  arms  a  single  1000  ohm  unit  tapped  at 
the  center  is  used.  The  rheostat  arm  units 
should  be  mounted  in  a  box  and  heavy  leads 
run  to  12  single  pole  switches  mounted  on  the 
top  of  the  box.  The  method  of  wiring  the  re- 


SELENIUM    CELL    CONSTRUCTION 


TESTING   AND    MATURING  39 

sistances  is  shown  in  Fig.  15,  the  switches  being 
marked  so  the  resistance  in  the  circuit  can  be 
quickly  determined.  With  this  arrangement 
any  resistance  from  100  to  111,000  ohms  can 
be  obtained  in  steps  of  100  ohms  by  opening 
the  proper  switches. 

The  ratio  arms  are  made  by  bringing  leads 
from  the  ends  and  center  of  the  1000  ohm  unit. 
Ratios  other  than  one  to  one  are  not  advised 
for  the  cells  are  so  sensitive  to  external  influ- 
ences that  one  system  of  measurement  must  be 
adopted  and  adhered  to  if  the  cells  are  to  be 
compared.  With  the  one  to  one  ratio  half  the 
current  flows  tnru  the  cells  when  the  bridge 
is  balanced. 

The  current  for  testing  the  cells  can  be  ob- 
tained from  a  dozen  three  cell  flashlight  bat- 
teries. The  battery  should  be  connected  to  a 
multipoint  switch  so  that  any  number  of  cells 
can  be  switched  into  the  current  as  desired. 

To  prepare  the  various  instruments  for  test- 
ing the  cells  with  the  bridge  circuit  connect  as 
shown  in  Fig.  16.  The  diagrammatic  wiring 
is  shown  in  the  insert  in  the  illustration.  The 
positive  or  carbon  of  the  battery  is  connected 
to  post  A.  The  source  of  alternating  current 
whether  from  the  lightning  mains  or  an  indue- 


SELENIUM    CELL    CONSTRUCTION 


TESTING    AND    MATURING  4! 

tion  coil  is  connected  to  the  double  pole  switch, 
current  for  the  lamp  in  the  enclosed  box  being 
supplied  to  it  thru  the  single  pole  switch. 

To  test  the  cell,  open  both  knife  switches  and 
place  switch  S  on  the  left  hand  point.  Place 
the  cell  in  the  clips  with  the  terminal  connected 
to  the  gold  leaf  in  the  clip  connected  to  binding 
post  A.  This  makes  the  gold  leaf  the  anode 
of  the  cell.  Now  close  the  battery  switch  ap- 
plying about  6  volts  to  the  bridge.  Balance 
the  bridge  by  adding  or  removing  resistance 
in  the  rheostat  arm  until  the  galvonometer 
gives  no  deflection.  The  resistance  of  the  cell 
in  the  dark  is  read  direct  from  the  markings  on 
the  open  switches  on  the  rheostat  box. 

Now  close  the  single  pole  knife  switch  light- 
ing the  cell  and  again  balance  the  bridge.  The 
latter  reading  will  be  the  light  resistance  of 
the  cell,  gold  anode. 

For  convenience  in  recording  the  data  a 
sheet  of  paper  should  be  ruled  into  columns 
headed,  date,  voltage  applied,  resistance  dark, 
resistance  lighted  (gold  anode),  resistance 
dark,  resistance  lighted  (copper  anode)  and 
remarks.  The  cells  should  be  numbered  and 
the  corresponding  data  sheet  headed  with  the 
same  number  so  that  changes  in  the  cell  can 


42  SELENIUM    CELL    CONSTRUCTION 

readily  be  detected  and  improvements  noted. 
A  separate  sheet  should  be  provided  for  each 
cell. 

Now  increase  the  voltage  to  9  volts  and  take 
another  set  of  readings  with  the  cell  both  dark 
and  lighted.  The  values  obtained  are  recorded 
and  the  voltage  increased  further.  The  cell  is 
tested  with  gradually  increasing  voltages  till 
the  maximum  voltage  possible  without  heating 
the  cell  has  been  applied.  Heating  of  the  cell 
can  usually  be  noted  by  a  faint  crackling  noise 
being  given  but  it  is  advisable  to  feel  the  sur- 
face of  the  mica  occasionally  to  detect  any 
heating. 

It  may  be  found  on  examining  the  data  ob- 
tained that  the  resistance  of  the  cell  has  varied 
with  different  voltages.  We  can  classify  the 
cells  by  calling  a  cell  in  which  the  resistance 
increases  with  an  increasing  voltage  the  A 
type,  those  in  which  the  resistance  falls  off 
with  an  increase  of  voltage  the  B  type  and 
those  in  which  changes  of  voltage  cause  little 
or  no  change  of  resistance  the  C  type. 

Now  reverse  the  cell  in  the  clips  making  the 
copper  plate  the  anode  and  repeat  the  above 
series  of  tests  and  record  the  values  in  the 
proper  columns  on  the  ruled  sheet.  It  will  be 


TESTING   AND    MATURING  43 

found  in  many  cases  that  the  reversal  of  the 
current  thru  the  cell  has  increased  the  resis- 
tance. A  cell  showing  this  characteristic  is 
polarized  while  one  in  which  the  reversal  of 
the  current  does  not  alter  the  resistance  to  any 
great  extent  is  non-polarized. 

The  cell  is  now  to  be  treated  with  alternat- 
ing current  for  a  period  of  five  minutes  by  open- 
ing the  battery  switch,  cutting  the  galvono- 
meter  out  of  the  circuit  and  closing  the  double 
pole  switch.  Current  will  flow  thru  the  ratio 
arms  and  the  cell.  Care  should  be  taken  that 
the  current  is  not  heavy  enough  to  heat  the 
cell  excessively.  If  this  occurs  reduce  the  cur- 
rent by  adding  resistance  till  heating  is  not 
detected. 

The  cell  is  again  tested  according  to  the  di- 
rections already  given  both  for  its  light  and 
dark  resistances  with  the  current  flowing  in 
both  directions  thru  the  cell.  If  the  difference 
between  the  dark  and  light  resistance  has  in- 
creased it  is  safe  to  say  that  the  cell  will  be 
sensitive  to  light  and  the  treatment  should 
be  continued  to  develop  this  property.  The 
quickest  method  of  doing  this  is  to  select  from 
the  various  readings  the  voltage  that  gives  the 
greatest  difference  in  the  dark  and  light  re- 


44  SELENIUM    CELL    CONSTRUCTION 

sistance.  Use  this  voltage  to  make  one  test 
after  each  treatment  with  A.  C.  till  the  sensi- 
tiveness of  the  cell  to  light  has  reached  a  fair^ 
value.  When  the  resistance  in  the  dark  is  ten 
times  as  great  as  in  the  light  the  cell  is  suitable 
for  experimental  working  and  treatment  can 
be  discontinued. 

Should  the  cell  have  practically  no  resistance 
when  first  put  in  the  testing  set  it  is  useless  to 
go  thru  the  entire  series  of  tests  till  the  resis- 
tance has  been  raised.  Treat  the  cell  with  A. 
C.  repeatedly  till  the  resistance  is  brought  up 
to  a  fair  amount,  at  least  500  ohms,  then  make 
the  tests  outlined  previously. 

Sometimes  even  prolonged  treatment  will 
fail  to  raise  the  resistance,  in  this  case  test  the 

f  }\ 

cell  for  polarization  by  reversing  in  the  clips. 
If  strongly  polarized  the  cell  may  be  a  good 
generator,  i.e.,  will  give  a  current  under  the 
influence  of  light.  To  test  this,  connect  the 
galvonometer  to  pests  A  and  D  of  the  set,  put 
battery  switch  on  left  hand  point  and  light  the 
cell  by  closing  the  single  pole  knife  switch.  If 
the  galvonometer  gives  a  deflection  it  indicates 
that  current  is  being  generated  in  the  cell. 
This  cell  should  be  reserved  for  use  as  a  gen- 
erator, this  property  being  increased  by  short 


TESTING    AND    MATURING  45 

circuiting  the  terminals  and  exposing  the  cell 
to  light  periodically  until  it  will  generate  a  fair 
amount  of  current.  The  current  generated  by 
these  cells  is  a  true  photo-electric  current,  no 
chemical  action  taking  place,  the  light  rays  be- 
ing converted  directly  into  electricity  by  some 
unknown  action  of  the  cell.  The  current  flows 
from  the  copper  plate  thru  the  external  circuit 
back  to  the  gold  foil  terminal. 

Should  the  cell  after  prolonged  treatment 
with  A.C.,  fail  to  increase  in  resistance  or  hav- 
ing increased  in  resistance  and  remain  insen- 
sitive to  light  and  not  prove  to  be  a  current 
generator  the  cell  must  be  classed  as  useless. 
It  is  seldom  indeed  that  this  occurs  for  a  cell 
that  shows  even  a  slight  change  in  resistance 
when  lighted  should  be  treated  with  A.C.  from 
time  to  time  and  used  in  experiments  and  will 
eventually  increase  in  sensitiveness  sufficiently 
to  be  of  value.  When  a  cell  proves  intracta- 
ble, the  gold  foil  may  be  carefully  removed  by 
means  of  a  stiff  brush  and  the  cell  put  back  in 
the  hot  press  to  be  reconstructed.  Retreating 
with  heat  and  pressure  will  often  give  a  very 
sensitive  cell. 

Even  when  the  cells  are  being  used  for  ex- 


46  SELENIUM    CELL    CONSTRUCTION 

perimental  work  the  maturing  process  will  be 
going  on.  The  point  of  maturity  and  maxi- 
mum sensitiveness  of  any  cell  has  been  reached 
when  the  dark  resistance  of  the  cell  remains 
constant  over  a  period  of  time.  The  maturing 
seems  to  affect  the  cell  by  increasing  the  dark 
resistance,  the  light  resistance  remaining  prac- 
tically the  same  thruout.  From  this  it  will  be 
seen  that  a  cell  with  a  very  low  resistance  when 
first  made  may  under  proper  treatment  become 
extremely  sensitive  to  light. 

As  previously  mentioned  the  testing  set  may 
be  used  for  measuring  the  resistance  of  the 
cells  by  the  substitution  method.  The  appara- 
tus is  connected  as  shown  in  Fig.  17  when  so 
used,  a  diagrammatic  circuit  being  given  in  the 
insert.  When  the  battery  switch  is  on  the  left 
hand  point  the  cell  is  in  series  with  the  battery 
and  galvonometen  When  the  switch  is  moved 
to  the  right  hand  point  the  rheostat  is  in  series 
with  the  battery  and  galvonometer.  The 
method  of  measuring  is  to  first  determine  the 
galvonometer  deflection  with  the  cell  in  the  cir- 
cuit and  then  by  switching  the  rheostat  into 
the  circuit  and  adjusting  it  so  the  galvonometer 
gives  the  same  deflection  enabling  the  resis- 
tance of  the  cell  to  be  read  directly.  The  same 


TESTING   AND    MATURING 


47 


48  SELENIUM    CELL    CONSTRUCTION 

series  of  tests  should  be  gone  through  with 
this  method  as  with  the  bridge  circuit. 

The  substitution  method  has  the  advantage 
of  indicating  roughly  the  sensitiveness  of  the 
cell  to  light.  With  the  cell  darkened  and  the 
current  flowing  thru  it  the  galvonometer  will 
give  a  certain  deflection,  on  lighting  the  cell 
the  deflection  will  increase  indicating  a  lower- 
ing of  the  resistance.  The  determination  of 
cell  sensitiveness  is  more  rapid  and  this  circuit 
can  be  employed  when  the  cell  has  a  low  resis- 
tance at  first  and  it  is  necessary  to  increase  it 
before  tests  can  be  made.  With  the  substi- 
tution method  of  measurement  it  is  advisable 
to  use  a  rather  insensitive  galvonometer  or 
meter  since  the  current  at  times,  may  reach  a 
value  that  would  damage  a  delicate  instrument. 
A  tangent  galvonometer  with  heavy  windings 
will  be  found  most  suitable. 

To  determine  if  a  cell  is  sensitive  enough  to 
close  a  relay  the  substitution  circuit  may  be 
used  with  a  relay  connected  in  place  of  the  gal- 
vonometer. If  the  cell  is  suitable  for  the  pur- 
pose the  relay  will  close  when  the  cell  is  lighted. 

For  practical  purposes  the  sensitiveness  of 
a  cell  to  light  and  its  current  generating  prop- 
erties are  the  only  ones  of  value  but  other  prop- 


TESTING    AND    MATURING 


49 


5O  SELENIUM    CELL    CONSTRUCTION 

erties  of  these  cells  will  be  mentioned  later. 
These  may  be  developed  by  proper  treatment 
and  the  results  of  experime'nts  along  these 
lines  can  hardly  be  foreseen  with  any  degree 
of  accuracy  at  the  present  stage  of  the  develop- 
ment of  the  cells. 

When  satisfied  that  a  cell  is  properly  made 
and  fairly  sensitive  it  can  be  permanently 
sealed  by  pouring  melted  sealing  wax  around 
the  edges  and  smoothing  with  a  hot  knife:  A 
small  sheet  of  paper  should  be  pasted  on  the 
back  of  the  cell  with  the  more  important  char- 
acteristics marked  thereon  as  well  as  the  most 
suitable  voltage.  The  terminal  connected  to 
the  gold  foil  can  be  indicated  by  a  positive  (  +  ) 
sign  for  convenience  in  properly  connecting 
the  cell  into  the  circuit. 


- 

CHAPTER  V 

APPLICATIONS  OF  SELENIUM  CELLS 

To  go  into  a  detailed  description  of  the 
many  applications  of  selenium  cells  is  not  pos- 
sible in  a  work  of  the  size  of  this.  However 
a  discussion  of  the  manifold  applications  will 
point  out  in  a  marked  manner  the  wonderful 
possibilities  of  a  perfected  selenium  cell. 

The  use  of  selenium  cells  for  photometric 
purposes  was  suggested  by  Clark  on  the  occa- 
sion of  the  first  announcement  of  the  light  sen- 
sitive properties  of  selenium.  It  was  hoped 
that  the  selenium  cell  would  remove  the  color 
stumbling  block  in  measuring  the  brilliancy  of 
light  but  to  date  a  successful  photometer  using 
selenium  has  not  been  produced.  The  pro- 
posed method  was  to  connect  a  selenium  cell 
in  series  with  a  galvonometer  properly  cali- 
brated and  to  allow  the  light  to  be  measured  to 
fall  on  the  selenium  cell,  its  brilliancy  in  candle- 
power  or  foot  candles  to  be  read  direct  from  the 
scale  on  the  galvonometer. 

51 


52  -SELENIUM    CELL   CONSTRUCTION 

A  cell  of  the  Fritts  type  with  a  gold  elec- 
trode would  not  be  suitable  for  the  purpose 
on  account  of  it  only  passing  the  green  rays 
but  Fritts  suggested  that  a  number  of  cells  be 
used,  one  with  a  silver  foil  electrode  to  pass 
blue  rays  and  so  on  to  include  all  the  colors  of 
the  spectrum.  Naturally  the  use  of  a  per- 
fectly transparent  conductor  on  the  surface  of 
the  cell  would  make  it  possible  to  use  one  cell 
for  the  entire  range  of  color  and  with  proper 
precautions  a  conducting  liquid  might  solve  the 
problem.  This  is  worthy  of  attention  and  ex- 
perimentx 

The  application  of  selenium  cells  to  the  trans- 
mission of  speech  over  a  beam  of  light  has 
received  mention  from  time  to  time  in  the  tech- 
nical press  principally  in  connection  with  the 
experiments  of  Bell  and  Taintor  or  Ruhmer. 
Ruhmer  has  succeeded  in  talking  a  distance  of 
4^  miles  using  a  speaking  arc  at  the  trans- 
mitting station.  The  arc  was  mounted  in  the 
focus  of  a  parabolic  mirror,  the  microphone 
being  connected  inductively  to  the  arc  circuit 
by  means  of  an  induction  coil.  The  beam  of 
light  was  picked  up  by  a  second  parabolic  mir- 
ror and  focused  on  a  selenium  cell  connected 
to  a  battery  and  telephone  receiver.  The  voice 


APPLICATIONS    OF    SELENIUM    CELLS         53 

waves  impinging  on  the  microphone  will  act 
to  vary  the  intensity  of  the  light  beam  sent  out 
by  the  arc  and  in  this  manner  affect  the  sele- 
nium cell  at  the  receiving  station  which  alters 
the  strength  of  the  current  thru  the  receivers 
and  thus  reproduces  the  words. 

For  experimental  purposes  a  small  incan- 
descent lamp  may  be  used  at  the  transmitting 
station  connected  in  series  with  a  telephone 
transmitter  and  battery.  The  receiving  end 
comprises  a  selenium  cell  with  proper  battery 
and  sensitive  telephone  receiver.  With  a  little 
care  in  adjustment  the  simple  arrangement 
will  transmit  the  voice  across  a  darkened  room 
without  difficulty. 

Another  application  of  selenium  made  by 
Ruhmer  is  the  automatic  control  of  light  buoys. 
When  such  buoys  are  placed  in  out  of  the  way 
places  it  has  been  necessary  to  leave  the  light 
burning  day  and  night.  To  prevent  the  fre- 
quent recharging  of  the  gas  reservoir  this 
method  required,  a  selenium  cell  was  arranged 
to  turn  the  gas  on  and  ignite  it  at  nightfall  and 
to  extinguish  it  again  in  the  morning.  The 
cell  is  connected  to  a  voltmeter  the  needle  of 
which  moves  between  two  contacts.  During 
the  day  the  needle  rests  against  one  contact  and 


54  SELENIUM    CELL    CONSTRUCTION 

the  gas  is  turned  off  by  an  electromagnet.  At 
nightfall  as  the  resistance  of  the  cell  increases 
the  needle  falls  back  till  it  touches  the  other 
contact  and  operates  an  electrical  mechanism 
for  turning  on  the  gas  and  igniting  it. 

Similar  applications  only  await  the  perfec- 
tion of  a  reliable  selenium  cell  in  quantities. 
They  could  be  used  to  automatically  control 
street  lights  and  lights  on  advertising  signs  lo- 
cated along  railroads  and  similar  routes  of 
travel. 

On  the  other  hand  they  could  be  employed 
in  connection  with  a  suitable  recording  instru- 
ment to  register  the  intensity  of  the  sunlight 
and  thus  serve  as  a  valuable  adjunct  to  the 
weather  bureau. 

They  have  been  used  to  a  limited  extent  by 
Prof.  Barnard  of  Lick  Observatory  in  connec- 
tion with  astronomical  work  and  by  Minchin 
in  an  automatic  detector  of  comets. 

The  oft  heralded  successful  transmission  of 
photographs  over  a  wire  has  yet  to  become  an 
accomplished  fact.  Photos  have  been  trans- 
mitted by  Korn  with  a  fair  degree  of  success 
using  selenium  cells  but  such  work  awaits  the 
perfect  cell.  Many  other  investigators  have 
attempted  to  solve  the  problem  and  have  pro- 


APPLICATIONS    OF   SELENIUM    CELLS         55 

posed  various  methods,  employing  images 
thrown  on  ground  glass  screen  or  thru  a  pho- 
tographic negative,  a  selenium  cell  or  a  bank 
of  cells  to  be  affected  by  the  high  and  low 
lights  of  the  picture.  At  the  receiving  end  the 
current  is  used  to  control  a  light  source  by  one 
of  the  many  possible  methods  to  reproduce  the 
picture  on  sensitized  paper.  These  inventions 
have  been  variously  termed  the  Telescope,  Tel- 
ectroscope,  Telephot,  Telephotograph,  etc. 
However  with  the  exception  of  a  very  few  they 
have  not  progressed  beyond  descriptive  mat- 
ter, drawings  and  the  most  preliminary  experi- 
ments. One  interested  in  the  subject  would 
do  well  to  obtain  copies  of  the  various  patents 
issued  on  the  art  thru  a  patent  lawyer  who  will 
do  this  for  a  nominal  sum. 

A  scheme  has  been  proposed  to  utilize  the 
cells  in  connection  with  motion  pictures  to  pro- 
duce the  so  called  talking  pictures.  This  seems 
entirely  feasible  provided  some  form  of  tele- 
phonic relay  is  used  to  amplify  the  reproduced 
voice  currents.  The  method  suggested  is  to 
print  the  spoken  words  on  the  motion  picture 
film  alongside  the  pictures  in  parallel  lines  of 
black  and  white.  To  do  this  a  beam  of  light 
15  focused  on  the  film  while  the  picture  is  being 


56  SELENIUM    CELL    CONSTRUCTION 

taken,  the  light  being  controlled  electrically  by 
microphones  concealed  near  the  actors.  The 
words  spoken  acting  to  vary  the  intensity  of 
the  light,  which  variations  will  be  printed  on 
the  film. 

In  reproducing  the  picture  a  light  is  ar- 
ranged to  pass  thru  the  voice  strip  and  fall  on 
a  selenium  cell.  In  this  manner  the  various 
shades  of  black  and  white  imprinted  on  the 
film  will  cause  variations  in  the  resistance  of 
the  cell  and  loud  talking  telephones  connected 
to  the  cell  thru  a  telephonic  relay  will  reproduce 
the  words.  This  method  has  the  decided  ad- 
vantage of  absolute  synchronism  lacking  in  all 
mechanical  devices  ever  developed  for  the  pur- 
pose. 

In  an  attempt  to  enable  the  blind  to  read 
printed  matter  by  sound  Prof.  F.  C.  Brown  has 
devised  what  is  termed  a  Phonoptican.  In  his 
device  three  or  four  tiny  selenium  cells  are 
mounted  in  a  row.  The  length  of  the  row  is 
equal  to  the  height  of  the  printed  letters.  Each 
cell  is  connected  to  a  telephone  receiver  thru 
a  separate  interrupter.  For  convenience  the 
telephone  receiver  used  contains  as  many  sep- 
arate coils  and  diaphragms  as  there  are  cells 
used.  It  will  be  clear  that  when  light  falls  on 


APPLICATIONS    OF    SELENIUM    CELLS         5/ 

one  of  the  cells  a  certain  note  will  be  heard  in 
the  receiver  and  after  a  little  practice  the  cell 
affected  can  be  instantly  determined. 

The  cells  are  mounted  in  a  tiny  box  and 
moved  over  a  brilliantly  illuminated  printed 
page.  The  cells  will  be  illuminated  and  dark- 
ened by  reflection  from  the  printed  letter  and 
in  a  certain  order  for  each  individual  letter. 
Since  each  cell  gives  a  different  sound  in  the 
telephone  receiver  each  letter  will  have  a  dif- 
ferent series  of  sounds.  A  sound  alphabet  is 
thus  made  that  after  a  little  practice  will  enable 
a  blind  person  to  read  printed  text. 

Hammond  of  Radio  control  fame  has  em- 
ployed selenium  cells  for  the  purpose  of  con- 
trolling boats  at  a  distance.  His  method  con- 
sists in  having  a  number  of  selenium  cells,  each 
being  sensitive  to  a  certain  colored  beam  of 
light  and  responding  to  that  color  only.  .  These 
cells  are  connected  to  separate  relays  that  con- 
trol the  various  functions  required  of  the  boat 
or  torpedo.  Thus  by  throwing  a  beam  of  light 
of  a  certain  color  on  the  cells  any  desired  relay 
can  be  closed  and  the  mechanism  controlled  at 
a  distance. 

The  so  called  Electric  Dog  constructed  by 
B.  F.  Meissner,  that  follows  a  light  carried  by 


58  SELENIUM    CELL    CONSTRUCTION 

a  person  employs  two  selenium  cells  located  be- 
hind condensing  lenses  with  an  opaque  plate 
between  them.  The  cells  are  mounted  in  a 
box  fitted  with  wheels,  driven  by  a  motor  and 
steered  by  an  electromagnetic  arrangement. 
The  cells  are  connected  to  relays  so  that  when 
light  falls  on  either  cell  the  motor  will  start 
and  the  device  will  turn  towards  the  lighted 
side.  This  results  in  the  box  turning  till  the 
other  cell  receives  an  equal  amount  of  light 
which  closes  the  other  relay  and  straightens 
out  the  steering  mechanism  to  head  the  ma- 
chine straight  for  the  light.  In  this  manner 
the  "Dog"  will  follow  a  flashlight  with  unerr- 
ing precision,  keeping  a  straight  course  when 
both  lenses  are  equally  illuminated  but  when 
one  receives  more  light  than  the  other  it  will 
turn  towards  the  lighted  side.  This  illustrates 
one  of  the  amusing  applications  to  which  sele- 
nium cells  may  be  adopted. 

In  cable  telegraphy  use  is  made  of  a  siphon 
recorder  or  reflecting  galvonometer.  The  lat- 
ter causes  a  beam  of  light  to  swing  to  one  side 
or  the  other  according  to  whether  a  dot  or  dash 
is  indicated.  By  mounting  a  selenium  cell 
either  side  of  the  center  of  the  swing  and  pro- 


APPLICATIONS   OF   SELENIUM    CELLS         59 

viding  stops  it  has  been  possible  to  actuate  re- 
lays and  print  the  dots  and  dashes. 

The  selenium  cell  is  particularly  adapted  to 
burglar  alarm  work.  By  arranging  a  cell  in 
places  to  be  protected  and  connecting  it  to  a 
battery  and  relay  an  alarm  can  be  turned  in 
should  an  intruder  flash  a  light  on  the  cell. 
This  method  has  the  advantage  of  not  warning 
the  intruder  that  an  alarm  has  been  given  which 
is  an  assurance  of  his  capture. 

The  above  covers  in  a  brief  form  the  more 
important  applications  of  selenium  cells  in  gen- 
eral, being  possible  with  most  types  of  cells 
that  are  sufficiently  sensitive.  However  the 
Fritts  cell  possesses  several  interesting  fea- 
tures aside  from  its  resistance  being  altered 
by  light. 

As  previously  mentioned  some  cells  are  cap- 
able of  generating  a  current.  To  what  extent 
this  property  can  be  developed  is  a  matter  of 
conjecture  only.  It  cannot  be  denied  that  they 
would  form  an  ideal  source  of  current  of  small 
values.  Compact,  sealed,  practically  unbreak- 
able and  perfectly  portable  are  but  a  few  good 
features  of  such  a  battery.  The  cells  are  com- 
paratively cheap  to  construct  and  once  made  re- 


/ 


60  SELENIUM    CELL    CONSTRUCTION 

quire  no  further  attention  beyond  reasonable 
care.  If  a  number  of  cells  were  well  developed 
they  might  prove  valuable  in  utilizing  Solar 
energy.  For  this  purpose  an  arrangement 
would  be  necessary  to  protect  them  from  heat, 
for  instance  an  alum  cell.  It  should  be  borne 
in  mind  that  this  is  a  direct  transformation  of 
light  energy  into  electrical  energy  without 
chemical  action  of  any  kind,  this  is  the  fore- 
runner of  the  inevitable  Solar  Generator. 

Another  strange  property  of  the  cells  is  their 
ability  to  produce  sounds  when  a  pulsating  cur- 
rent is  passed  thru  them.  Under  certain  con- 
ditions a  telephonic  current  may  cause  them  to 
reproduce  speech  when  connected  to  a  micro- 
phone and  battery.  Weak  sounds  are  also 
produced  when  an  interrupted  beam  of  light 
is  allowed  to  fall  on  a  short  circuited  cell. 

It  was  also  mentioned  by  Fritts  that  his  cells 
change  in  sensitiveness  with  the  kind  of  bat- 
tery employed  with  them.  They  were  found 
rather  insensitive  with  bichromate  of  potash 
cells,  sensitive  with  Leclanche  cells  and  ex- 
tremely sensitive  when  another  current  gener- 
ating cell  was  employed  as  a  source  of  current. 
This  fact  opens  a  wide  field  for  experiment. 
Perhaps  a  certain  type  of  battery  will  result  in 


APPLICATIONS    OF    SELENIUM    CELLS         6l 

making  the  selenium  cells  extremely  sensitive 
and  staple.  Conjecture  on  the  subject  is  with- 
out bounds.  Just  what  causes  a  cell  to  change 
in  sensitiveness  with  different  current  sources? 
Is  there  some  unrecognized  force  in  one  cur- 
rent that  is  not  found  in  the  other  ?  The  solv- 
ing of  these  problems  may  not  be  work  of  the 
easiest  or  the  soonest  done  but  the  accomplish- 
ment is  worth  the  labor. 


CHAPTER  VI 

THE    CARE    OF    SELENIUM    CELLS 

A  SELENIUM  cell  will  give  much  better  serv- 
ice and  have  a  longer  life  if  a  little  care  is  taken 
with  it.  These  suggestions  if  followed  will 
prevent  to  a  great  extent  irreparable  damage 
to  the  cells. 

ixKeep  them  cool.  Do  not  allow  them  to. be- 
come heated  to  any  extent  or  the  gold  foil  will 
combine  with  the  selenium  to  form  a  gold  sel- 
enide  and  destroy  the  sensitiveness  of  the  cell. 
This  condition  will  be  indicated  by  dark  brown 
splotches  on  the  face  of  the  cell. 

''Use  as  small  current  values  as  possible  to 
accomplish  a  desired  result.  This  demands 
the  use  of  high  resistance  relays,  resistances 
may  be  employed  in  the  circuit  to  limit  the  cur- 
rent but  a  high  resistant  relay  is  the  proper 
solution. 

/Use  the  proper  voltage  as  determined  by  the 
testing  set. 

l/Do  not  expose  light  sensitive  cells  to  bright 

light  for  excessively  long  periods  of  time.     It 

62 


THE    CARE   OF   SELENIUM    CELLS  63 

fatigues  the  cell  <and  causes  it  to  become  insen- 
sitive temporarily. 

When  using-  generator  cells  do  not  pass  a 
current  from  an  outside  source  thru  them  while 
lighted.  This  destroys  temporarily  the  gen- 
erating powers  of  the  cell. 

Keep  the  cells  dry.  If  not  of  the  sealed  type 
keep  them  in  box  having  a  few  lumps  of  cal- 
cium chloride  in  the  bottom  to  absorb  moisture. 
-^When  not  in  use  they  should  be  kept  in  a 
light  tight  box  -but  should  be  exposed  to  light 
each  day  or  so  regardless  of  whether  they  are 
used  or  not.  This  aids  maturing  and  retains 
the  sensitiveness  of  the  cells. 
.  Should  their  resistance  drop  to  any  great  ex- 
tent after  continued  use  it  can  again  be  raised 
by  treating  with  an  alternating  or  pulsating 
current  as  described  under  treating  and  test- 
ing cells. 


THE  END 


Date  Due 


Library  Bureau  Cat.  No.  1137 


415285 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


